Today's wind turbines, in particular large scale wind turbines with power outputs in the scale of above 1 MW, are very complex systems. Despite their large size, their operational state needs to be adaptable to current weather conditions, in particular wind conditions. For that purpose, the position of the rotor blades of the rotors of such wind turbines can be adapted. A so-called pitch control system allows for positioning the rotor blades against the wind by rotating the blades around their longitudinal axis. Thus, the rotational speed of the rotor can be controlled and a maximum power output can be achieved.
The usual way of pitch control of the rotor blades is by using an electric pitch control system in which electric engines control the pitch of the blades. However, it has been wished for to use hydraulic pitch systems (or pneumatic pitch systems—which are also summarized under the expression “hydraulic pitch system” in the context of this application) rather than electric ones. Such hydraulic systems are often easier to control and they also still function in the case of an interruption of power output of the generator of the wind turbine because they are not directly dependent on electric power supply by the wind turbine itself. In order to drive such hydraulic pitch systems it is necessary to have a transport system which transports a hydraulic and/or pneumatic fluid (such as hydraulic oil, water or any other liquid or gas) into the pitch control system in the hub under a certain pressure. In other words, the hydraulic and/or pneumatic fluid is put under a certain pressure by means of a pump and lead to a distribution block, to blade blocks and accumulator blocks which are all located inside the hub in close proximity to the rotor blades.
The transport of this pressurised hydraulic and/or pneumatic fluid, however, has proven to be quite complicated. This is due to the fact that the hub rotates in operation of the wind turbine so that a solution has to be found of how the pipes of the transport system are not rotated together with the hub in such a way that they will be damaged due to torsions.
This problem is even increased if the wind turbine is realized as a direct drive wind turbine with a drive train directly connecting the rotor with a generator. The drive train then comprises those parts which project from the hub into the nacelle and which are essentially formed pipe-like. Therefore, such drive train in a direct drive wind turbine can also be characterized as a communication link or communication assembly in contrast to drive trains in an indirect drive wind turbine (where the drive train comprises a number of shafts). Such direct connection between the rotor and the generator means that no gearbox which could be used to transfer the (rather slow) rotation of the rotor into a faster rotation of a shaft. Rather, the rotation of the rotor of the wind turbine is directly transferred into the generation of electric energy.
Whereas it is possible to lead pipes of a transport system for hydraulic and/or pneumatic fluids through the drive train, i.e. a shaft, of an indirect-drive wind turbine with a gearbox, such shaft does not exist in direct drive wind turbines. Therefore, the problem of transferring the hydraulic and/or pneumatic fluid from the nacelle into the hub is particularly difficult to solve in the case of direct drive wind turbines.